This invention applies generally to the transmission of analog signals via sampling and digitization and, more specifically, to broadband transmission applications such as the digitization and transport of broadband system return path signals.
In certain instances, it is desirable to represent and transmit a continuously varying electronic waveform, i.e., an analog signal, as a series of discrete binary words, i.e., in a digital representation. These binary words may then be transferred to a receiving location using a suitable transmitter and receiver combination and converted back to an analog format. In the return path of a cable television system, for example, analog information is typically generated by subscribers and received at a nodal station, i.e., a node, as an analog, radio frequency (RF) electrical signal. The analog return path signal can then be converted to serial digital format at the node and transmitted along a fiber optic cable to an optical receiver located at the headend station. At the headend, the digital information is converted back to an analog waveform for processing.
Electronically, an analog-to-digital (A/D) converter at the transmitter converts the continuously-varying input waveform to a series of digital values. A/D converters measure the amplitude of an analog input signal at equally-spaced moments in time, and output lines of the A/D converter are turned on and off to generate a binary number that approximates the measured value. This digital word is transmitted by electro-optical means to a receiving location. Such means commonly include transmission at optical or radio-frequency (RF) wavelengths. At the receiver, a digital-to-analog (D/A) converter inverts the digitizing process to generate an analog waveform from the series of transported binary words.
In an ideal case, the analog signal reconstructed by the receiver D/A converter would be identical to the input of the A/D converter. Unfortunately, the output word size of the transmitter A/D converter is limited to an integral number of bits, M. Since M output bits can only represent 2M discrete values, and since an analog input has an infinite number of amplitude states, there is an unavoidable quantization error associated with the digitization process. In a digital transmission system, the total noise is the sum of the quantization errors and the distortion introduced by the A/D converter and the D/A converter.
In prior art communication systems, quantization errors could be minimized by increasing the resolutions of the A/D and D/A converters to generate and transmit digital words having a greater number of bits. In such a situation, though, the communication channel will quickly become xe2x80x9ccloggedxe2x80x9d unless greater numbers of channels are provided, such as by installing more communication cables, which is a very expensive undertaking. Because each communication channel, or medium, can only transmit a given amount of information within a given time frame, transmission of digital words having greater numbers of bits is prohibitively expensive, and thus impractical, in current communication systems. Alternatively, more expensive lasers and photodiodes could be used to send bits at higher data rates, but this option is also much more expensive than permitted in current systems. Furthermore, bandwidth limitations of the communication channel could still create problems associated with transmission of even the higher data rate signals by the more expensive lasers and photodiodes.
To overcome this problem, greater resolution A/D converters can be used to generate digital words having greater numbers of bits, then the digital words can be truncated. For example, the output of a 12-bit A/D converter could be truncated to an 8-bit size required by the transmitter by throwing away (truncating) the last four bits of the A/D converter output. Truncation or any other method of reducing the number of bits representing the sample, however, always increases the quantization error for one or more possible amplitudes in the analog input range. As a result, the dynamic range and performance of many digital transmission systems are predominantly limited by the width of their transport channels, not by the digitization and reconstruction processes.
In order to improve the dynamic performance of channel limited communications systems, an improved method for decreasing word length is desired.